Method and apparatus for producing a sheet of a material containing alkaloids

ABSTRACT

The invention relates to a method for producing a sheet of a material containing alkaloids, the method comprising: —mixing a material containing alkaloids with water ( 6 ) to form a slurry ( 11 ); —forming a sheet ( 10 ) from the slurry ( 11 ); —compressing the sheet ( 10 ) between a first ( 307 ) roller and a second ( 308 ) roller, said rollers ( 307, 308 ) forming a gap ( 311 ) therebetween where the sheet ( 10 ) is inserted, to form a compressed sheet ( 10 ) having a desired thickness (t); and —changing the diameter ( 17 ) of the first roller ( 307 ) in order to change the desired thickness (t) of the compressed sheet ( 10 ). The invention also relates to an apparatus for producing a sheet ( 10 ) of a material containing alkaloids.

This invention relates to a casting apparatus and method for producing a cast web of a material containing alkaloids.

In particular, the material containing alkaloids is homogenized tobacco material, preferably used in an aerosol-generating article such as, for example, a cigarette or a “heat-not-burn” type tobacco containing product.

Today, in the manufacture of tobacco products, besides tobacco leaves, also homogenized tobacco material is used. This homogenized tobacco material is typically manufactured from parts of the tobacco plant that are less suited for the production of cut filler, like, for example, tobacco stems or tobacco dust. Typically, tobacco dust is created as a side product during the handling of the tobacco leaves during manufacture.

The most commonly used forms of homogenized tobacco material are reconstituted tobacco sheet and cast leaf (TCL is the acronym for tobacco cast leaf). The process to form homogenized tobacco material sheets commonly comprises a step in which tobacco dust and a binder, are mixed to form a tobacco slurry. The slurry is then used to create a tobacco web, for example by casting a viscous slurry onto a moving metal belt to produce so called cast leaf. Alternatively, a slurry with low viscosity and high water content can be used to create reconstituted tobacco in a process that resembles paper-making. Once prepared, homogenized tobacco webs may be cut in a similar fashion as whole leaf tobacco to produce tobacco cut filler suitable for cigarettes and other smoking articles. A process for making such homogenized tobacco is for example disclosed in European Patent EP 0565360.

In a “heat-not-burn” aerosol-generating article, an aerosol-forming substrate is heated to a relatively low temperature, in order to form an aerosol but prevent combustion of the tobacco material. Further, the tobacco present in the homogenized tobacco material is typically the only tobacco, or includes the majority of the tobacco, present in the homogenized tobacco material of such a “heat-not burn” aerosol-generating article. This means that the aerosol composition that is generated by such a “heat-not burn” aerosol-generating article is substantially only based on the homogenized tobacco material. Therefore, it is important to have good control over the composition of the homogenized tobacco material, for the control for example, of the taste of the aerosol.

Due to variations in the physical properties of the slurry, for example, consistency, viscosity, fibre size, particle size, moisture or the age of the slurry, standard casting methods and apparatus may result in unintended variations in the application of the slurry onto a support during the casting of web of homogenized tobacco. A non-optimal casting method and apparatus may lead to inhomogeneity and defects of the cast web of homogenized tobacco.

An important parameter of the cast sheet is its thickness, which is preferably as homogeneous as possible so that the users' smoking experience can be substantially the same using any final product obtained embedding the cast sheet. Variations in the thickness, even minimal variations, may result in products that need to be discarded, enhancing costs and production time.

In known processes, the thickness of the sheet is determined by a casting blade, which casts the sheet onto a conveyor belt, and the distance between the blade and the belt substantially determines the thickness of the sheet. Any imperfection in the blade, in the conveyor belt or in their alignment may cause the production of an uneven sheet.

Further, changes in the desired thickness of the cast sheet requires a careful and slow realignment and movements of the casting blade, which take time and often lead to machine stops before reaching the desired new thickness.

There is therefore the need for a method and an apparatus to obtain a cast sheet of a material containing alkaloids having a substantially uniform thickness which also allow relatively rapid thickness' changes.

The invention relates to a method for producing a sheet of a material containing alkaloids, the method comprising: mixing a material containing alkaloids with water to form a slurry; forming a sheet from the slurry; compressing the sheet between a first roller and a second roller, said first roller and second roller forming a gap therebetween where the sheet is inserted, to form a compressed sheet having a desired thickness; and changing the diameter of the first roller in order to change the desired thickness of the compressed sheet.

In the method of the invention, the thickness of the sheet is controlled by a compression step between rollers. As soon as the sheet is formed, for example by casting or by extrusion, the sheet is compressed between a first couple of rollers to obtain the desired thickness of the sheet. In case it is desired to change the thickness of the sheet, or in case the resulting measured thickness is not the desired one, the apparatus can rapidly adjust to the new desired thickness changing the diameter of the first roller so that the compression felt by the sheet changes. The diameter of both rollers can be changed as well. The process is relatively simple, but an accurate control of the thickness is obtained, which also allows on line changes.

As used herein, the terms “sheet” denotes a laminar element having a width and length substantially greater than the thickness thereof. The width of a sheet is preferably greater than about 10 millimetres, more preferably greater than about 20 millimetres or about 30 millimetres. A continuous “sheet” is herein called “web”. Even more preferably, the width of the sheet of material containing alkaloids is comprised between about 60 millimeters and about 2500 millimeters. As used herein, the term “casting blade” denotes a longitudinally shaped element that may have an essentially constant cross-section along major parts of its lengthwise extension. It shows at least one edge that is intended to come into contact with a pasty, viscous or liquid-like substance to be influenced by said edge, such as a slurry. Said edge may have a sharp and knife-like edge. Alternatively, it may have a rectangular or a rounded edge.

As used herein, the term “movable support” denotes any means comprising a surface that can be moved in at least one longitudinal direction. The movable support may form a closed loop so as to provide an uninterrupted transport in one direction. The movable support may include a conveyor belt. The movable support may be essentially flat and may show a structured or an unstructured surface. The movable support may have no openings on its surface or may include orifices, preferably of such a size that they are impenetrable for the slurry deposited on it. The movable support may comprise a sheet-like movable and bendable band. The band may be made of a metallic material, including but not limited to steel, copper, iron alloys and copper alloys, or of a rubber material. The band may be made of a temperature-resistant material so that it can be heated to speed up the drying process of the slurry.

As used herein, the term “slurry” denotes a liquid-like, viscous or pasty material that may comprise an emulsion of different liquid-like, viscous or pasty material and that may contain a certain amount of solid-state particles, provided that the slurry still shows a liquid-like, viscous or pasty behaviour.

A “material containing alkaloids” is a material which contains one or more alkaloids. The alkaloids may comprise nicotine. The nicotine may be found, for example, in tobacco.

Alkaloids are a group of naturally occurring chemical compounds that mostly contain basic nitrogen atoms. This group also includes some related compounds with neutral and even weakly acidic properties. Some synthetic compounds of similar structure are also termed alkaloids. In addition to carbon, hydrogen and nitrogen, alkaloids may also contain oxygen, sulfur and, more rarely, other elements such as chlorine, bromine, and phosphorus.

Alkaloids are produced by a large variety of organisms including bacteria, fungi, plants, and animals. They can be purified from crude extracts of these organisms by acid-base extraction. Caffeine, nicotine, theobromine, atropine, tubocurarine are examples of alkaloids.

As used herein, the term “homogenised tobacco material” denotes material formed by agglomerating particulate tobacco, which contains the alkaloid nicotine. The material containing alkaloids can thus be a homogenized tobacco material.

The most commonly used forms of homogenized tobacco material is reconstituted tobacco sheet and cast leaf. The process to form homogenized tobacco material sheets commonly comprises a step in which tobacco dust and a binder, are mixed to form a slurry. The slurry is then used to create a tobacco web. For example by casting a viscous slurry onto a moving metal belt to produce so called cast leaf. Alternatively, a slurry with low viscosity and high water content can be used to create reconstituted tobacco in a process that resembles paper-making.

In the method of the invention, a slurry is formed. The slurry contains a material containing alkaloids and water. It may also preferably comprise a binder and an aerosol former. It may also include cellulose fibers in addition to those contained in the material containing alkaloids.

The slurry may comprise a number of additional different components or ingredients. These components may influence the properties of the cast web of material containing alkaloids. A first ingredient is the material containing alkaloids, for example in powder form. This material can be for example a tobacco powder blend, which preferably contains the majority of the tobacco present in the slurry. The tobacco powder blend is the source of the majority of tobacco in the homogenized tobacco material and thus gives the flavour to the final product, for example to an aerosol produced heating the homogenized tobacco material. A cellulose pulp containing cellulose fibres is preferably added to the slurry in order to increase the tensile strength of the alkaloids material web, acting as a strengthening agent.

A binder is preferably added as well, in order to enhance the tensile properties of the homogenized sheet. An aerosol-former may be added to promote the formation of aerosol. Further, in order to reach a certain viscosity and moisture optimal for casting the web of material containing alkaloids, water may be added to the slurry.

The quantity of binder added to the slurry may be comprised between about 1 percent and about 5 percent in dry weight of the slurry. More preferably, it is comprised between about 2 percent and about 4 percent. The binder used in the slurry may be any of the gums or pectins described herein. The binder may ensure that the powder of the material containing alkaloids remains substantially dispersed throughout the homogenized web. Although any binder may be employed, preferred binders are natural pectins, such as fruit, citrus or tobacco pectins; guar gums, such as hydroxyethyl guar and hydroxypropyl guar; locust bean gums, such as hydroxyethyl and hydroxypropyl locust bean gum; alginate; starches, such as modified or derivitized starches; celluloses, such as methyl, ethyl, ethylhydroxymethyl and carboxymethyl cellulose; tamarind gum; dextran; pullalon; konjac flour; xanthan gum and the like. The particularly preferred binder for use in the present invention is guar.

The introduction of cellulose fibres in the slurry typically increases the tensile strength of the web of material containing alkaloids, acting as a strengthening agent. Therefore, adding cellulose fibres may increase the resilience of the web of material containing alkaloids. Cellulose fibres for including in a slurry for webs of material containing alkaloids are known in the art and include, but are not limited to: soft-wood fibres, hard wood fibres, jute fibres, flax fibres, tobacco fibres and combination thereof. In addition to pulping, the cellulose fibres might be subjected to suitable processes such as refining, mechanical pulping, chemical pulping, bleaching, sulphate pulping and combination thereof. Cellulose fibres may include tobacco stem materials, stalks or other tobacco plant material. Preferably, cellulose fibres such as wood fibres comprise a low lignin content. Alternatively, fibres, such as vegetable fibres, may be used either with the above fibres or in the alternative, including hemp and bamboo. The length of cellulose fibres is advantageously between about 0.2 millimetres and about 4 millimetres. Preferably, the mean length per weight of the cellulose fibres is between about 1 millimetre and about 3 millimetres. Further, preferably, the amount of the cellulose fibres is comprised between about 1 percent and about 7 percent in dry weight basis of the total weight of the slurry (or homogenized tobacco sheet).

The mean length of the fibers refers to their real length (regardless whether they are curled or have kinks) as measured by MORFI COMPACT commercialised by Techpap SAS. The mean length is the mathematical mean of the measured length of the fibers by MORFI COMPACT over a measurement of N fibers, where N>5. The MORFI COMPACT is a fiber analyser that measures the length of the fibers following the framework of the fibers, thus measuring their real developed length. Measured objects are considered fibers if their length is comprised between 200 microns and 10000 microns and their width is comprised between 5 microns and 75 microns. Fibers length is measured when deionized water is added to the fibers and Morfi software is used.

Suitable aerosol-formers for inclusion in slurry for sheet of material containing alkaloids such as homogenised tobacco material are known in the art and include, but are not limited to: monohydric alcohols like menthol, polyhydric alcohols, such as triethylene glycol, 1,3-butanediol and glycerine; esters of polyhydric alcohols, such as glycerol mono-, di- or triacetate; and aliphatic esters of mono-, di- or polycarboxylic acids, such as dimethyl dodecanedioate and dimethyl tetradecanedioate.

Examples of preferred aerosol-formers are glycerine and propylene glycol.

The slurry may have an aerosol-former content of greater than about 5 percent on a dry weight basis. The slurry may have an aerosol former content of between about 5 percent and about 30 percent by weight on a dry weight basis. More preferably, the aerosol-former is comprised between about 10 percent to about 25 percent of dry weight of the slurry. More preferably, the aerosol-former is comprised between about 15 percent to about 25 percent of dry weight of the slurry.

The binder and the cellulose fibres are preferably included in a weight ratio comprised between about 1:7 and about 5:1. More preferably, the binder and the cellulose fibres are included in a weight ratio comprised between about 1:1 and about 3:1.

The binder and the aerosol-former are preferably included in a weight ratio comprised between about 1:30 and about 1:1. More preferably, the binder and the aerosol-former are included in a weight ratio comprised between about 1:20 and about 1:4.

Preferably, the alkaloid containing material is tobacco. The binder and the tobacco particles are preferably included in a weight ratio comprised between about 1:100 and about 1:10. More preferably, the binder and the tobacco particles are included in a weight ratio comprised between about 1:50 and about 1:15, even more preferably between about 1:30 and 1:20.

The aerosol-former and the tobacco particles are preferably included in a weight ratio comprised between about 1:20 and about 1:1. More preferably, the aerosol-former and the tobacco particles are included in a weight ratio comprised between about 1:6 and about 1:2.

The aerosol former and the cellulose fibres are preferably included in a weight ratio comprised between about 1:1 and about 30:1. More preferably, the aerosol-former and the cellulose fibres are included in a weight ratio comprised between about 5:1 and about 15:1.

The cellulose fibres and the tobacco particles are preferably included in a weight ratio comprised between about 1:100 and about 1:10. More preferably, the cellulose fibres and the tobacco particles are preferably included in a weight ratio comprised between about 1:50 and about 1:20.

Further, from the slurry, a sheet is formed. To form the sheet, a sheet former is preferably used. In order to form the sheet, the slurry can be for example casted, preferably on a movable support, along a casting direction. The slurry may be contained in a casting box having an aperture at the bottom and a casting blade. The casting-box is preferably box-shaped.

For casting, as a sheet former, casting blade may be used. The casting blade is preferably arranged perpendicular to the casting direction. The web of material may be formed by means of the casting blade that casts the slurry present in the casting box. The slurry for example falls by gravity from the casting box and comes into contact with the casting blade. The edge of the casting blade forms a gap with the surface of the movable support and the slurry passes through the aperture defined by said gap.

The slurry might be extruded in order to form the sheet. The sheet former may be thus an extruder. Therefore, the sheet exits an extruder where it is preferably compressed and heated. Also in this case, the slurry is preferably extruded onto a movable support. Any process to form the sheet may be used in this invention, that is, any sheet-former device can be considered.

The direction along which the sheet is extruded or casted defines the transport direction of the sheet. In order to form a continuous sheet or web of material containing alkaloids, the sheet—while formed—is preferably moved so that it can continuously be formed creating a web. Preferably, the sheet is moved along the transport direction by the movable support.

The formed sheet is then compressed between two rollers, which form a first couple or pair of rollers. The rollers of the first couple are called first roller and second rollers. The first roller and second roller form a first gap therebetween, in which the sheet is inserted and compressed. Preferably, the thickness of the sheet after being compressed by the first couple of rollers is smaller than the thickness the sheet had before being compressed by the first couple of rollers.

Preferably, the first roller and second rollers have a cylindrical shape and have a first axis of rotation and a second axis of rotation. Preferably, the first axis of rotation and second axis of rotation are parallel to each other. Preferably, the first axis of rotation and second axis of rotation are perpendicular to the transport direction of the sheet. For example, the first axis of rotation and second axis of rotation are parallel to the width of the sheet.

Before the compression by the first couple of rollers, the humidity of the sheet—substantially just formed—is preferably relatively high. The water content of the sheet just before the compression between the first roller and second roller of the first couple is preferably comprised between about 60 percent and about 85 percent of the total weight of the sheet. Preferably, the water content of the sheet just before the compression between the first roller and second roller is comprised between about 65 percent and about 80 percent of the total weight of the sheet. More preferably, it is comprised between about 70 percent and about 78 percent. The first couple of rollers is preferably positioned directly in front of the sheet former, such as the extruder or the casting blade, without any other element therebetween.

Before the compression of the sheet by the first couple of rollers, the sheet has an initial thickness, called also first thickness. The initial thickness is preferably comprised between about 0.2 millimetres and about 2 millimetres. More preferably, the initial thickness is comprised between about 0.4 millimetres and about 1 millimetres. Even more preferably, the initial thickness is comprised between about 0.5 millimetres and about 0.8 millimetres.

After the compression by the first couple of rollers, the initial thickness of the sheet is preferably decreased, and the initial thickness of the sheet becomes a second thickness after the first couple of rollers.

In case it is desired to have a sheet with a different second thickness, or if the second thickness is not the desired one, for example due to a change in parameters of the overall process (for example a less dense slurry, or a different slurry composition are used, etc.) the size of the gap between the first roller and second roller is varied. This variation is performed by changing the dimension of the diameter of the first roller of the first couple. The diameters of both first roller and second roller may be changed as well.

The change of diameter of the first roller means that the first roller has its diameter changed, by any means, so that the gap between the first roller and second roller can change. The first roller remains the same, only its diameter is changed. In the diameter change step, the first roller remains rotatably fixed to the apparatus. This allows a diameter change also while the compression of the sheet is performed. On-line changes of the gap between the first roller and second roller are thus possible.

Preferably, the step of changing the diameter of the first roller includes: changing the diameter of the first roller while rotating the first roller. Production is not interrupted.

The first roller, the diameter of which is changed, can be also not in direct contact to the sheet. The sheet can be compressed between the first roller and second roller, but between the first roller and the sheet a further element is inserted. This further element can be a further roller.

This change may be performed on line, without the need of stopping the production, because the variation of the diameter of the first roller may not need a change of the rollers or any interruption in production. The change in diameter can be performed while the sheet is compressed. A reduction or an increase in the gap between the two rollers of the first couple leads to a reduction or an increase, respectively, in the sheet' second thickness.

According to the method of the invention, an easy and quick variations of the thickness of the sheet can be obtained, without machine stop. Further, the variations can be in any direction, that is, with this diameter variation it is possible to make the sheet thicker or thinner. The variations can also be automatic, the only manual operation being inputting a different second thickness' value in a feedback system. A sensor may check the second thickness of the sheet after the first couple of rollers. A comparison may be made with the measured second thickness and desired thickness. In case the measured second thickness does not match the desired one, the gap between the rollers is changed, varying the diameter of the first roller. The diameters of the first roller and of the second roller can be varied as well.

Preferably, changing the diameter of the first roller includes changing a width of the gap. Preferably, changing the diameter of the first roller implies a change in the width of the gap, which can be wider or smaller than it was before the change. In this way, a thicker or thinner sheet can be formed.

Preferably, changing the diameter of the first roller includes inflating or deflating the first roller. “Inflate” means the process of increasing the size of the roller by a fluid filling the inside of the roller. The fluid may be pressurized air. The rollers may be formed in a material which allows a deformation in case it is inflated or deflated, for example using a pressurized fluid. The material in which the first roller is formed may be an elastic material. The rollers can be “tyre-like” so that a change in their diameter is possible by increasing or decreasing their internal pressure. In this way, a very efficient change of the gap's width can be achieved, in particular a relatively fast one.

Preferably, changing the diameter of the first roller includes changing the temperature of the first roller. As known, several materials may change their dimensions with temperature. Commonly, higher the temperature, higher is the volume occupied by the material. Preferably, the material in which the first roller is formed has a high termal expansion. Preferably, the higher the temperature is, the larger the diameter of the first roller becomes.

Preferably, at the beginning of the step of compressing the sheet, a water content of the sheet is comprised between about 60 percent and about 85 percent of the total weight of the sheet. The step of compressing the sheet between the first roller and the second roller is performed preferably when the sheet is “just formed”, for example just cast, and therefore with a high moisture content. In this way, the thickness of the cast sheet can be better regulated because the slurry is still pliant and soft, easily compressible.

Preferably, the method includes the step of changing the diameter of the first roller as a function of a desired thickness of the sheet containing alkaloids.

Preferably, a feedback loop is present in the apparatus of the invention so that there is a constant comparison between the actual thickness of the sheet and the desired thickness. When variations of the thickness takes place, the diameter of the roller is changed to keep obtaining the desired thickness. For example, the thickness of the sheet after the first couple of rollers may be measured and the diameter of the first roller may be changed if the measured thickness is different than the desired thickness. More preferably, the diameter of the first roller may be changed if the measured thickness is outside a pre-determined range of thicknesses. The pre-determined range of thicknesses may be a range centered around the desired thickness.

Further, if a sheet having a different thickness than the one produced at a given moment is desired, it is possible to modify the size of the gap between the first roller and second roller so as to obtain the new desired thickness without any machine interruption, simply changing the diameter of the first roller. Preferably, the method includes the step of drying the sheet during the compression step between the first roller and second roller. Preferably, while the thickness of the sheet is regulated by the compression, the sheet is also dried. Therefore, preferably the first couple of rollers is contained in a dryer. Preferably, the drying is achieved by a combination of hot roller surfaces in direct contact with the sheet and hot air. The first roller and second roller define an external surface. Preferably, one of the first or second roller, or both, is heated up by a hot fluid such as steam or vapour so that its external surface becomes hot. Preferably, both the temperature of the hot roller surface in contact with the drying sheet and the temperature of the hot air are comprised between about 40 degrees Celsius and about 250 degrees Celsius.

Preferably, the compression step by the first couple of rollers also improves the efficiency of the drying step. Generally, drying is performed by a hot fluid. The compression may squeeze some water out of the sheet and therefore the overall drying takes less time, or a lower temperature hot fluid can be used for drying.

Preferably, the method includes the step of regulating a temperature of the first roller or of the second roller. The drying may be further improved in efficiency by heating up the rollers. Alternatively, the rollers can be cooled, for example the temperature of the couple of rollers close to an exit of the dryer can be reduced. Preferably, the temperature of the rollers for heating or cooling is comprised between about 10 degrees Celsius and about −250 degrees Celsius.

Preferably, the method includes the step of further compressing the sheet compressed by the first roller and second roller between a third roller and fourth roller. Preferably, after the first compression, a second compression by a second couple of rollers takes place according to the invention. The second compression is performed by a third roller and a fourth roller, which preferably form a second gap therebetween, where the sheet is introduced and compressed by the second couple of rollers.

The second compression preferably takes place downstream the first compression in the transport direction of the sheet.

Preferably, the third roller and fourth roller have a cylindrical shape and have a third axis of rotation and a fourth axis of rotation. Preferably, the third axis of rotation and fourth axis of rotation are parallel to each other. Preferably, the third axis of rotation and fourth axis of rotation are perpendicular to the direction of movement of the sheet. For example, the third axis of rotation and fourth axis of rotation are parallel to the width of the sheet. Therefore, preferably, the first axis of rotation, second axis of rotation, third axis of rotation and fourth axis of rotation are all parallel to each other.

After the compression by the second couple of rollers, the thickness of the sheet is further reduced from the second thickness to a third thickness. After the second couple of rollers, that is, after the second compression by the second couple of rollers, the third thickness of the sheet is preferably comprised between about 0.5 millimetres and about 0.05 millimetres. The third thickness of the sheet is more preferably comprised between about 0.3 millimetres and about 0.1 millimetres. The third thickness is the final desired final thickness of the sheet. The final thickness, the third thickness, of the sheet is therefore preferably obtained in a multi-step process. Any number of couple of rollers can be used. A better control of the final thickness is therefore obtained, because the rollers dimensions can be easily controlled and further “small” unevenness obtained in the first compression might be corrected in the second compression.

More than two couples of rollers can be considered in the method of the invention. An even more precise control of the final thickness of the sheet can be obtained. Therefore, from the first thickness at forming to the third—final —thickness, the sheet may have many intermediate thicknesses. Reaching the final thickness in several steps allows a very precise control of the homogeneity of the sheet itself. In the following, N couple of rollers, where N≥2, are considered. The first couple of rollers is considered to be the closest to the sheet former, while the second couple of rollers is the last couple of rollers and N−2 additional couples of rollers are placed between the first couple and the second couple.

Preferably, in case of N rollers, the pressure applied to the sheet by the couple of rollers increases from the first couple of rollers to the second couple of rollers (=last couple of rollers in the row) and monotonously increases in the N−2 couple of rollers positioned therebetween along the direction of movement of the sheet.

Preferably, the step of forming a sheet includes the step of casting a sheet. Preferably, the step of forming a sheet includes the step of extruding the sheet. The sheet can be formed by any known method. The invention can be applicable to any forming system or method to form sheet from a slurry.

Preferably, the method includes the step of drying the sheet during the compression step between the first couple of rollers or during the compression step between the second couple of rollers or between the compression step between the first couple of rollers and the compression step between the second couple of rollers. Preferably, while the thickness of the sheet is regulated by the several-steps compression, the sheet is also dried. Therefore, preferably the N couple of rollers are contained in a dryer. Preferably, the drying is achieved by a combination of hot roller surfaces in direct contact with the sheet and hot air. The rollers are heated up by a hot fluid such as steam or vapour. Preferably, both the temperature of the hot roller surface in contact with the drying sheet and the temperature of the hot air are comprised between about 40 degrees Celsius and about 250 degrees Celsius. Preferably, in case of N couple of rollers, all rollers are included in the dryer. Therefore, preferably the drying step takes place during each of the N compression steps and also while the sheet moves from one couple or rollers to the next couple of rollers.

Preferably, the compression step by the couple of rollers also improves the efficiency of the drying step. Generally, drying is performed by a hot fluid. The compression may squeeze some water out of the sheet and therefore the overall drying takes less time, or a lower temperature hot fluid can be used for drying.

Preferably, the method includes the step of: inserting a rigid element between said first roller and the sheet; and the step of changing the diameter of the first roller includes: changing the pressure exerted by the first roller onto the rigid element. The first roller may not be in direct contact to the sheet. A rigid element, such as a roller and more preferably a roller the diameter of which is fixed and not changeable, may be interposed between the sheet and the first roller. The first roller, changing diameter, pushes the rigid element towards or away the sheet, so that the compression on the sheet changes. It is preferred to insert a rigid element between the first roller and the sheet to keep constantly a surface which is locally flat in contact to the sheet. When the first roller inflates or deflates, its outer surface may deform from a cylindrical surface. Therefore, interposing a rigid element assures that the surface in contact to the sheet is always the same, only the applied pressure changes.

The invention also relates to an apparatus for producing a sheet of a material containing alkaloids, comprising: a tank adapted to contain a slurry formed by a material containing alkaloids and water; a sheet forming device to form a sheet from the slurry; a first roller and a second rollers, said first roller and second roller forming a gap therebetween where the sheet is inserted, to compress the sheet; a diameter changing device to change the diameter of the first roller.

Many advantages of the invention have been stated previously and not repeated herewith. The apparatus of the invention includes a first couple of rollers, for example included in a dryer. The compression of the sheet between the rollers may alter the thickness of the sheet. A change in the first roller's diameter may therefore change the thickness of the sheet. A change of both rollers' diameter can be envisaged as well; in this case each roller includes a diameter changing device, or the same diameter changing device is active on both first and second roller.

Preferably, said diameter changing device includes a pressurized fluid supply. Preferably, said rollers are inflatable, like tyres, so they include an external deformable shell, and thus a pressurized fluid supply can be used to alter their diameter blowing up the shell.

Preferably, a width of said first roller or second roller is at least double a width of said tank. The width of the first roller or second roller is defined as the dimension of the first roller or of the second roller along its axis of rotation. The width of the tank is the dimension of the tank along the same axis of rotation. Preferably, the axis of rotation is perpendicular to the transport direction of the sheet. Due to the fact that a change in the diameter of the roller may slightly alter the roller's shape, in particular its flatness, it is preferred that the width of the roller is “much bigger” than that of the sheet (which often corresponds or it is function of the width of the tank) so that the sheet can be compressed in the flattest portion of the roller, which is generally the central part. It is thus preferable to have a first roller or a second roller much larger than the tank for the slurry, which defines the width of the sheet, so that it is possible to compress the sheet in the central part of the first roller or second roller.

Preferably, the sheet forming device includes a casting apparatus. Preferably, the sheet forming device includes an extruding apparatus. Any sheet forming apparatus can be used in the present invention, casting and extruding being the most used and optimal to obtain a sheet of material containing alkaloids.

Preferably, the apparatus includes a third roller and a fourth roller forming a second gap therebetween wherein the sheet can be inserted, the third roller and fourth roller being positioned downstream the first roller and second roller in the direction of movement of the sheet.

Preferably, the second gap is smaller than the first gap. In case of N couple of rollers, the first gap of the first couple of rollers closest to the forming device is the biggest in width and the second gap of the second couple of rollers the smallest. The N−2 remaining rollers have a gap the width of which is comprised between the first gap and the second gap.

Preferably, the first couple of rollers includes a first roller and a second roller and the second couple of rollers includes a third roller and a fourth roller, and wherein the diameter of the first roller is bigger than the diameter of the third roller. In case of N rollers, preferably the diameter of the rollers decreases along the direction of movement of the sheet. A better control of the thickness of the sheet is obtained. Preferably, the decrease in the diameter of the rollers in turn determines a decrease of the contact surface between the rollers and the sheet and a more accurate thickness regulation and control is achieved.

Preferably, the width of the gap between the roller is decreased moving the rollers close to each other.

Preferably, the first couple of rollers includes a first roller and a second roller and the second couple of rollers includes a third roller and a fourth roller, and an external surface of the third roller has a higher hardness than a surface of the first roller. Hardness is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. Some materials are harder than others. Depending on the material, the rollers may have a different hardness. Hardness in case of steel rollers is preferably comprised between about 1 and about 50 HRC (Rockwell scale), the hardness in case of plastic rollers is preferably comprised between about D10 and about D100 (shore durometer), the hardness in case of rubber rollers is preferably comprised between about A10 and about A100 (shore durometer). The rollers can be formed in metal, plastic or rubber. The surface of the rollers can be coated with layers of different materials having a different hardness. Preferably, in case of N couple of rollers, the hardness of the couple of rollers increases from the first couple of rollers towards the Nth couple of rollers in the direction of movement of the sheet (transport direction).

Preferably, the apparatus includes a thickness sensor to measure a thickness of the sheet, said diameter changing device changing the diameter of the of the first or of the second roller on the basis of an output signal of said thickness sensor. A feedback loop is preferably present to elaborate the signals coming from the sensor and alter the diameter of the roller accordingly.

Preferably, the apparatus also comprises a movable support, the movable support being driven by a first or second roller of the first couple of rollers. Preferably, a movable support is present to transport the sheet along a transport direction. Preferably, the belt is driven by one of the roller of the first couple. Preferably, the movable support ends after the first couple of rollers. Preferably, in case of N pair of rollers, the movable support extends in the transport direction of the sheet passing through a given number of rollers. Preferably, after the first couple of rollers or second couple of rollers, the sheet is “solid” so it is self-sustaining and it may driven by motorized couple of rollers through the next rollers. Preferably, the movable support ends between the first and the second couple of rollers.

Preferably, the apparatus includes a rigid element positioned in the gap between the first roller and second roller, so that the sheet is inserted between the second roller and the rigid element. The gap between the first roller and second roller is “big” so that another element, such as an additional roller, can be interposed. Variations in the diameter of the first roller change the pressure exerted by the rigid element onto the sheet.

Specific embodiments will be further described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 shows a flow diagram of a method to produce slurry for homogenized tobacco material according to the invention;

FIG. 2 shows a block diagram of a variant of the method of FIG. 1;

FIG. 3 shows a block diagram of a method for production of a homogenized tobacco material according to the invention;

FIG. 4 shows an enlarged view of one of the steps of the method of FIG. 1, 2 or 3;

FIG. 5 shows an enlarged view of one of the steps of the method of FIG. 1, 2 or 3;

FIG. 6 shows a schematic view of an apparatus for performing the method of FIGS. 1 and 2;

FIG. 7 shows a schematic view of an apparatus for performing the method of FIG. 3; and

FIG. 8 shows a schematic front view of a different embodiment of a detail of the apparatus of the invention.

With initial reference to FIG. 1, a method for the production of a sheet of material containing alkaloids, in the present example a homogenized tobacco sheet, from a slurry according to the present invention is represented. The first step of the method of the invention is the selection 100 of the tobacco types and tobacco grades to be used in the tobacco blend for producing the homogenized tobacco material. Tobacco types and tobacco grades used in the present method are for example bright tobacco, dark tobacco, aromatic tobacco and filler tobacco.

Only the selected tobacco types and tobacco grades intended to be used in the production of the homogenized tobacco material undergo the processing according to following steps of the method of the invention.

The method includes a further step 101 in which the selected tobacco is laid down. This step may comprise checking the tobacco integrity, such as grade and quantity, which can be for example verified by a bar code reader for product tracking and traceability. After harvesting and curing, the leaf of tobacco is given a grade, which describes the stalk position, quality, and colour.

Further, the lay down step 101 might also include, in case the tobacco is shipped to the manufacturing premises for the production of the homogenized tobacco material, de-boxing or case opening of the tobacco boxes. The de-boxed tobacco is then preferably fed to a weighing station in order to weight the same.

Moreover, the tobacco lay down step 101 may include bale slicing, if needed, as the tobacco leaves are normally transported in bales when boxed and shipped.

The tobacco bales are separated depending on the tobacco type. For example there may be a processing line for each tobacco type. Therefore, the following steps are performed for each tobacco type, as detailed below. These steps may be performed subsequently per grade such that only one production line is required. Alternatively, the different tobacco types may be processed in separate lines. This may be advantageous where the processing steps for some of the tobacco types are different. For example, in conventional primary tobacco processes bright tobaccos and dark tobaccos are processed at least partially in separate processes, as the dark tobacco often receives an additional casing. However, according to the present invention, preferably, no casing is added to the blended tobacco powder before formation of the homogenized tobacco web.

Further, the method of the invention includes a step 102 of coarse grinding of the tobacco leaves.

According to a variant of the method of the invention, after the tobacco lay down step 101 and before the tobacco coarse grinding step 102, a further shredding step is performed, not depicted in the drawings. In the shredding step, the tobacco is shredded into strips having a mean size comprised between about 1 millimetre and about 100 millimetres.

Preferably, after the shredding step, a step of removal of non-tobacco material from the strips is performed (not depicted in FIG. 1).

Subsequently, the shredded tobacco is transported towards the coarse grinding step 102. The flow rate of tobacco into a mill to coarse grind the strips of tobacco leaf is preferably controlled and measured.

In the coarse grinding step 102, the tobacco strips are reduced to a particle size of between about 0.25 millimetres and about 2 millimetres. At this stage, the tobacco particles are still with their cells substantially intact and the resulting particles do not pose relevant transport issues.

With size of the particle of the material containing alkaloids, the Dv95 size is meant. Each of the values above listed indicates the Dv95 of the particle size. The “v” in Dv95 means that a volume distribution is considered. The use of volume distributions introduces the concept of the equivalent sphere. An equivalent sphere is a sphere which is equal to the real particle in the property which we are measuring. Thus for light scattering methods, it is a sphere which would produce the same scattering intensities as the real particle. This is substantially a sphere having the same volume of the particle. Further, “95” in Dv95 means the diameter where ninety-five percent of the distribution has a smaller particle size and five percent has a larger particle size. Thus the particle size is that size according to a volume distribution where 95 percent of the particles have a diameter (of the corresponding sphere having substantially the same volume of the particle) smaller than the stated value. A particle size of 60 microns means that 95 percent of the particles have a diameter smaller than 60 microns, where the diameter is the diameter of the sphere having a corresponding volume than the particle.

Preferably, after the coarse grinding step 102, the tobacco particles are transported, for example by pneumatic transfer, to a blending step 103. Alternatively, the step of blending 103 could be performed before the step of coarse grinding 102, or where present, before the step of shredding, or, alternatively, between the step of shredding and the step of coarse grinding 102.

In the blending step 103, all the coarse grinded tobacco particles of the different tobacco types selected for the tobacco blend are blended. The blending step 103 therefore is a single step for all the selected tobacco types. This means that after the step of blending there is only need for a single process line for all of the different tobacco types.

In the blending step 103, preferably mixing of the various tobacco types in particles is performed.

After the blending step 103, a fine grinding step 104 to a tobacco powder size of between about 0.03 millimetres and about 0.12 millimetres is performed. This fine grinding step 104 reduces the size of the tobacco down to a powder size suitable for the slurry preparation. After this fine grinding step 104, the cells of the tobacco are at least partially destroyed and the tobacco powder may become sticky. Powder size is the Dv95 size as detailed above.

The so obtained tobacco powder can be immediately used to form the tobacco slurry. Alternatively, a further step of storage of the tobacco powder, for example in suitable containers may be inserted (not shown).

With now reference to FIG. 2, a method of the invention for a manufacture of a homogenized tobacco web is shown. From step 104 of fine grinding, the tobacco powder is used in a subsequent slurry preparation step 105. Prior to or during the slurry preparation step 105, the method of the invention includes two further steps: a pulp preparation step 106 where cellulose fibres 5 and water 6 are pulped to uniformly disperse and refine the fibres in water, and a suspension preparation step 107, where an aerosol-former 7 and a binder 8 are premixed. Preferably, the aerosol-former 7 includes glycerol and the binder 8 includes guar. Advantageously, the suspension preparation step 107 includes premixing guar and glycerol without the introduction of water.

The slurry preparation step 105 preferably comprises transferring the premix solution of the aerosol-former and the binder to a slurry mixing tank and transferring the pulp to the slurry mixing tank. Further, the slurry preparation step comprises dosing the tobacco powder blend into the slurry mixing tank with pulp, and the guar—glycerol suspension. More preferably, this step also includes processing the slurry with a high shear mixer to ensure uniformity and homogeneity of the slurry.

Preferably, the slurry preparation step 105 also includes a step of water addition, where water is added to the slurry to obtain the desired viscosity and moisture.

In order to form the homogenized tobacco web, preferably the slurry formed according to step 105 is transported to a casting box where it is mixed and then it is cast in a casting step 108. Preferably, this casting step 108 includes transporting the slurry to a casting station and casting the slurry into web on a support. Preferably, during casting, the cast web thickness, moisture and density are controlled immediately after casting and more preferably are also continuously monitored and feedback-controlled using slurry measuring devices during the whole process.

A desired thickness of the sheet is preferably selected.

The homogenized cast web is then dried in a drying step 111 comprising a uniform and gentle drying of the cast web, for example in an endless, stainless steel belt. The endless, stainless steel belt may comprise individually controllable zones. Preferably, the drying step comprises monitoring the cast leaf temperature at each drying zone to ensure a gentle drying profile at each drying zone and heating the support where the homogenized cast web is formed. Preferably, the drying profile is a so called TLC drying profile.

During the drying step 111, a first compression step 109 takes place. The first compression step takes place when the sheet is on the belt. The compression is realized between a first couple of rollers that form a first gap therebetween, where the sheet is inserted and compressed. After the first compression, the sheet might be removed from the belt so that the sheet is afterwards free-standing. After the compression step 109, the gap between the rollers can be modified in case the resulting thickness of the sheet is not the desired thickness. Therefore a step in which the diameter of one of the rollers, or both, is changed takes place, step 110.

In a preferred embodiment, the sheet also undergoes a second step of compression, subsequent to the first step 109, in step 110 a, also between two rollers forming a second gap therebetween. Preferably, the second gap is smaller than the first. This second compression preferably takes place while drying also takes place. At the end of the compression steps, the desired thickness of the sheet is obtained. Preferably, also the second gap can be changed, changing the diameter of the second couple of rollers. This thickness of the sheet may be further changed due to the drying process.

At the conclusion of the web drying step 111, a monitoring step (not shown) is executed to measure the moisture content and number of defects present in the dried web.

The homogenized tobacco web that has been dried to a target moisture content is then preferably wound up in a winding step 112, for example to form a single master bobbin. This master bobbin may be then used to perform the production of smaller bobbins by slitting in a small bobbin forming process. The smaller bobbins may then be used for the production of an aerosol-generating article (not shown).

In case a sheet having a different thickness is desired in a further process, the distance between the rollers used in the first compression steps, second compression steps and third compression steps may be changed, that is, the width of the first gap, second gap or third gap may be varied in order to alter the thickness of the sheet after the drying step 111.

The method of production of a slurry for the homogenized tobacco material according to FIG. 1 is performed using an apparatus for the production of a slurry 200 depicted schematically in FIG. 3. The apparatus 200 includes a tobacco receiving station 201, where accumulating, de-stacking, weighing and inspecting the different tobacco types takes place. Optionally, in case the tobacco has been shipped into cartons, in the receiving station 201 removal of cartons containing the tobacco is performed. The tobacco receiving station 201 also optionally comprises a tobacco bale splitting unit.

In FIG. 3 only a production line for one type of tobacco is shown, but the same equipment may be present for each tobacco type used in the homogenised tobacco material web according to the invention, depending on when the step of blending is performed. Further the tobacco is introduced in a shredder 202 for the shredding step. Shredder 202 can be for example a pin shredder. The shredder 202 is preferably adapted to handle all sizes of bales, to loosen tobacco strips and shred strips into smaller pieces. The shreds of tobacco in each production line are transported, for example by means of pneumatic transport 203, to a mill 204 for the coarse grinding step 102. Preferably a control is made during the transport so as to reject foreign material in the tobacco shreds. For example, along the pneumatic transport of shredded tobacco, a string removal conveyor system, heavy particle separator and metal detector may be present, all indicated with 205 in the appended drawing.

Mill 204 is adapted to coarse grind the tobacco strips up to a size of between about 0.25 millimetres and about 2 millimetres. The rotor speed of the mill can be controlled and changed on the basis of the tobacco shreds flow rate.

Preferably, a buffer silo 206 for uniform mass flow control, is located after the coarse grinder mill 204. Furthermore, preferably mill 204 is equipped with spark detectors and safety shut down system 207 for safety reasons.

From the mill 204, the tobacco particles are transported, for example by means of a pneumatic transport 208, to a blender 210. Blender 210 preferably includes a silo in which an appropriate valve control system is present. In the blender, all tobacco particles of all the different types of tobacco which have been selected for the predetermined blend are introduced. In the blender 210, the tobacco particles are mixed to a uniform blend. From the blender 210, the blend of tobacco particles is transported to a fine grinding station 211.

Fine grinding station 211 is for example an impact classifying mill with suitable designed ancillary equipment to produce fine tobacco powder to the right specifications, that is, to a tobacco powder having a Dv95 size between about 0.03 millimetres and about 0.12 millimetres. After the fine grinding station 211, a pneumatic transfer line 212 is adapted to transporting the fine tobacco powder to a buffer powder silo 213 for continuous feed to a downstream slurry batch mixing tank 214 where the slurry preparation process takes place.

The slurry which has been prepared using the tobacco powder above described in steps 100-105 of the method of the invention is preferably also cast in a casting station 300 as depicted in FIG. 4.

Slurry from a buffer tank (not shown), is transferred by means of suitable pump with precision flow rate control measurement to the casting station 300. Casting station 300 comprises preferably the following sections. A precision slurry casting box and knife assembly 301 where slurry is cast onto a support 303, such as a stainless steel belt with the required uniformity and thickness for proper web formation, receives the slurry from the pump. A main dryer 302, having drying zones or sections is provided to dry the cast tobacco web. Preferably, the individual drying zones have steam heating on the bottom side of the support with heated air above the support and adjustable exhaust air control. Within the main dryer 302, the homogenized tobacco web is dried to desired final moisture on the support 303.

With now reference to the more detailed FIG. 5, the movable support 303 comprises a continuous stainless steel belt including a drum assembly. The precision slurry casting box and knife assembly 301 comprises the casting blade 304 and the casting box 305. Preferably, the steel belt 303 is wound around a pair of opposite drums 306, 307. The slurry is casted on the steel belt—at the drum 306—through the casting blade 304, which creates a continuous sheet 10 of homogenized tobacco material.

The casted slurry 10 is driven by the steel belt 303 along a casting direction indicated with an arrow 24 in FIG. 5 and enters the dryer 302, where it is progressively heated and homogeneously dried. In FIG. 5, the dryer 302 is only partially depicted.

The incoming slurry 11 is introduced into the casting box 305 from an inlet (not depicted), in particular a pipe, connected to a side wall 14 of the casting box 305, which puts this incoming slurry 11 close to the bottom of the casting box 305.

Slurry 11 from buffer tanks (not shown in the drawings) is transferred into the casting box 305 usually by means of a pump (not shown in the drawings). Preferably, the pump comprises a control (not visible in the drawing) of flow rate to control the amount of slurry 11 introduced in the casting box 305. The pump is advantageously designed to ensure that slurry transfer times are kept to the minimum necessary.

The amount of slurry 11 in the casting box 305 has a pre-determined level, which is preferably kept substantially constant or within a given range. In order to keep the amount of slurry 11 substantially at the same level, the pump controls the flow of slurry 11 to the casting box 305.

The casting blade 304 is associated to the casting box 305 in order to cast the slurry. The casting blade 304 has a dominant dimension which is its longitudinal width. The casting blade defines a first axis which is positioned along its longitudinal direction.

Between the casting blade 304 and the steel belt 303 a gap is present, the dimensions of which determine—among others—the initial thickness of the cast web 10 of homogenized tobacco material, at casting, called initial thickness. This initial thickness is preferably checked, for example by means of suitable sensor 15 (see FIG. 4) which has preferably a feedback loop with the casting blade 304. The gap formed between the casting blade and the steel belt may be modified on the basis of the signals outputted by sensor 15.

Casting blade 304 and belt 303 face each other and the belt is partially positioned below the casting blade 304. The drum 306 transports belt 303 preferably rotates in the directions depicted by arrows 24 and 26.

Casting station 300 also includes a first couple of rollers 310, formed by the second drum 307 as a first roller and a second roller 308. The first roller 307 and second roller 308 form a first gap 311 therebetween. Casting station 300 also comprises a control unit 400 and an actuator 19 connected to the first couple of rollers 310 to change the gap 311 formed therebetween.

The belt is also wound around the second drum 307, having diameter 17. The second drum 307 forms part of the first couple of rollers 310, first roller 307 being the second drum and the second roller 308, having diameter 18, being positioned vertically above the first roller 307. The two rollers form the first gap 311 therebetween, having a changeable thickness. The first couple of rollers 310 is positioned inside dryer 302. The sheet is inserted into the gap 311 and compressed, so that water is removed from the sheet. The thickness of the sheet after the first couple of rollers 310 is called first thickness and it is indicated with t₁.

The gap 311 can be changed modifying the diameter of the rollers, either diameter 17 of the first roller 307 or both diameters 17, 18 of first and second roller 307, 308. In order to modify the diameter 17 or 18, preferably a sensor 16 detects the thickness of the sheet downstream the first couple of rollers 310 in the direction of transport of the sheet and, if the thickness does not match the desired thickness, or a change in thickness is desired, a feedback loop activates an actuator 19 to change the diameter 17 or 18.

For example, control unit 400 can receive signals from sensor 16 regarding the thickness of the sheet 10 and activate actuator 19 in case the thickness measurements are not the desired one. Alternatively, the actuator 19 can be activated by control unit 400 if a change in thickness is desired.

The change in diameter is represented in FIG. 7. The roller 307 varies its diameter, for example it expands increasing its diameter from diameter 17 to diameter 17′, becoming first roller 307′. Arrows 20 show the uniform expansion of the surface of the roller, caused for example by a pressurized fluid introduced inside the roller 307.

The first couple of rollers 310 can be a first of a series of N couples of rollers, where N≥2. In FIG. 6 an example where N=3, where there is the first couple of rollers 310, a second couple of rollers 312 (the last couple of rollers before the sheet exits the dryer) and a third couple of rollers 313 positioned in between the first couple of rollers and the second couple of rollers. The second couple of rollers is formed by a third roller 314 and a fourth roller 315 forming a second gap 316 therebetween. Third roller 314 has diameter 24 and second roller 315 has diameter 25. The thickness of the sheet after the second couple of rollers is called second thickness and indicated with t₂. The third couple of rollers, interposed between the first couple of rollers 310 and the second couple of rollers 312, is formed by a fifth roller 317 and a sixth roller 318 forming a second gap 319 therebetween. Fifth roller 317 has diameter 27, and sixth roller 318 has diameter 28. The thickness of the sheet after the third couple of rollers is called third thickness and indicated with t₃. Each couple of rollers defines a gap between the rollers forming the couple. The width of the gaps between the rollers of the couples monotonously decreases from the first couple of roller to the second roller. This means that first gap 311 is wider than the third gap 319, which is in turn wider than the second gap 316. In the same manner, the first thickness of the sheet 10 diminishes from the thickest t₁ after the first couple of rollers 310 to the thinnest t₂ after the second couple of rollers 312.

In other words t₁>t₃>t₂. Preferably, all rollers positioned below the sheet 10 of all couples 310, 312, 313 can change their diameter. The diameter 17 of the first roller 317 may be changed. Diameter 27 of fifth roller 317 may be changed, diameter 24 of third roller 314 may be changed.

Preferably, the diameters of the rollers also diminishes from the first couple of rollers 310 till the second couple of rollers 312 (which have the smallest diameter). The third couple of rollers 313 has an intermediate diameter between that of the first couple of rollers and second couple of rollers.

The thickness t₂ of the sheet after the second couple of rollers 312 is preferably checked, for example by means of a suitable sensor 16 positioned downstream the dryer 302 (see FIGS. 4 and 5) in the direction of movement of the belt 303. A feedback loop is preferably present between the sensor 16 checking thickness t₂ and the first gap 311, second gap 316 and third gap 319 between the first couple of rollers 310, second couple of rollers 312 and third couple of rollers 313, respectively. These gaps 311, 316, 319 can be adjusted according to signals sent by sensor 16. The thickness of the sheet may be monitored in different locations. In FIG. 8, a different embodiment of the invention is shown. The first couple of rollers 310 includes not only the first roller 307 and second roller 308, but also an additional rigid roller 500 in contact to the sheet 10. The sheet 10 is therefore in contact with the additional rigid roller 500 and the second roller 308. When the first roller 307 changes its diameter, for example by inflating or deflating, the pressure applied by rigid roller 500 onto the sheet 10 changes as well. The rigid roller 500 is positioned inside first gap 311.

Downstream dryer 302, the dried sheet can be wound in a bobbin (not shown) to be stored and further used to produce aerosol generating articles. 

1. Method for producing a sheet of a material containing alkaloids, the method comprising: mixing a material containing alkaloids with water to form a slurry; forming a sheet from the slurry; compressing the sheet between a first roller and a second roller, said first roller and second roller forming a gap therebetween where the sheet is inserted, to form a compressed sheet having a desired thickness; and changing the diameter of the first roller in order to change the desired thickness of the compressed sheet.
 2. Method according to claim 1, wherein changing the diameter of the first roller includes changing a width of the gap.
 3. Method according to claim 1, wherein changing the diameter of the first roller includes inflating or deflating the first roller.
 4. Method according to claim 1, wherein at the beginning of the step of compressing the sheet, a water content of the sheet is comprised between about 60 percent and about 85 percent of the total weight of the sheet.
 5. The method according to claim 1, including the step of: changing the diameter of the first roller or of the second roller as a function of a desired thickness of the sheet containing alkaloids.
 6. The method according to claim 1, including the step of: drying the sheet during the compression step between the first roller and second roller.
 7. The method according to claim 1, including the step of: regulating a temperature of the first roller or of the second roller.
 8. The method according to claim 1, including the step of: further compressing the sheet compressed by the first roller and second roller between a third roller and fourth roller.
 9. The method according to claim 1, including the step of: inserting a rigid element between said first roller and the sheet; and wherein the step of changing the diameter of the first roller includes: changing the pressure exerted by the first roller onto the rigid element.
 10. Apparatus for producing a sheet of a material containing alkaloids, comprising: a tank adapted to contain a slurry formed by a material containing alkaloids and water; a sheet forming device to form a sheet from the slurry; a first roller and a second roller, said first roller and second roller forming a gap therebetween where the sheet is inserted to compress the sheet; a diameter changing device to change the diameter of the first roller.
 11. Apparatus according to claim 10, wherein said diameter changing device includes a pressurized fluid supply.
 12. Apparatus according to claim 10 any of claim 10, wherein the sheet forming device includes a casting apparatus or an extruding apparatus.
 13. Apparatus according to claim 10, including a third roller and a fourth roller forming a second gap therebetween wherein the sheet can be inserted, the third roller and fourth rollers being positioned downstream the first and second roller in the direction of movement of the sheet.
 14. Apparatus according to claim 10 3, including a thickness sensor to measure a thickness of the sheet, said diameter changing device changing the diameter of the first roller or of the second roller on the basis of an output signal of said thickness sensor.
 15. Apparatus according to claim 10, including a rigid element positioned in the gap between the first roller and second roller, so that the sheet is inserted between the second roller and the rigid element. 